This invention relates to mechanically strong and leak-free sealing of bodies comprising a tube of fused silica and a high temperature material of an electrical feed through, such as are used as flash lamps and laser lamps, and in particular to the construction of the ends of such lamps and a method of effecting the sealing of electrodes into the ends thereof.
According to UK published patent application GB 23 08 226 or U.S. Pat. No. 5,979,187, flash and laser lamps are generally constructed from a tube of fused silica/quartz, the opposite ends of which contain metal electrodes to which electrical operating power is supplied via conductive supports, which also serve to mount the lamp in a lamp holder, when in use.
Due to the different coefficients of expansion of metal and fused silica/quartz, special materials have been developed for interposing between the metal conductive supports for the electrodes and the tube wall of such lamps, to accommodate the differential rates of expansion, as the lamp in use increases and decreases in temperature. Typically, the electrodes are constructed from tungsten and an intermediate sleeve of a glass-like material having an appropriate coefficient of expansion, such as a sealing glass, is formed around the tungsten rod before it is introduced into and sealed to an end of the lamp tube. Sealing glass is supplied, inter alia, by Schott Glass Ltd., and GS10 sealing glass as supplied by Schott Glass has been used with quartz and tungsten combinations.
As used herein, the expression GS is intended to mean any suitable material which can be bonded to a metal electrode and likewise fused to fused silica/quartz materials and whose coefficient of expansion is such as to accommodate the generally greater expansion of metal (for a given temperature rise), than is produced in fused silica/quartz by the same rise in temperature. GSiO sealing glass is an example of a GS material.
The constructional steps of the known method leading to the formation of a glass to metal seal at an end or a fused silica/quartz tube are as follows:                (1) A tungsten electrode is prepared to receive a sleeve of GS sealing glass, by heating and rotation about its longitudinal axis;        (2) A stick of GS is also heated and, as the end of the stick becomes molten, it is brought into contact with the rotating heated tungsten rod support which extends axially from the tungsten electrode, so that molten glass becomes attached to and “smeared” over the surface of the rod to form a relatively uniform thickness sleeve over approximately 1-2 cm of the length of the rod;        (3) The central region of the sleeve is increased in thickness by reheating it and the GS stick, and while the sleeved rod is rotated, touching the end of the glass stick against the central region of the sleeve to cause an annular build-up of GS to occur. This step is commonly referred to as “spinning a bead” onto the sleeve;        (4) Next, a fused silica/quartz tube, cut to the desired length of the lamp housing, is heated at one end, while being rotated around its longitudinal axis, and the heated end is closed by spinning a bead of molten GS into and over the heated end of the tube. (The GS stick is of course heated before it is brought into contact with the heated end of the tube);        (5) One end of a smaller diameter tube of fused silica/quartz is then heated, the interior of the lamp tube is pressurised with a non-oxidizing gas, typically and usually nitrogen, and a region of the wall thereof is heated until soft to permit the heated end of the smaller diameter tube to be pushed therethrough and fused thereto, so as to extend radially as a side tube therefrom. By pushing the end of the smaller diameter side tube through the locally heated, softened region of the lamp tube wall, the interior of the latter communicates with the interior of the side tube, and this communication is maintained by maintaining a positive gas pressure in the lamp tube while the fusing is completed. After this the heat is removed;        (6) The end of the radially protruding side tube which has just been added is now closed by heating the outboard end thereof to collapse the side tube wall;        (7) The previously closed end of the fused silica/quartz lamp tube is now reheated, and the internal pressure of the assembly of tubes is increased, so as to cause the GS dome, which has closed the heated lamp tube end, to balloon axially and puncture;        (8) While rotating the lamp tube and keeping the punctured end hot and near molten, a carbon tool is introduced into the punctured end, and the diameter of the opening in the GS dome is made concentric with the lamp tube axis and enlarged, so as to be capable of receiving the electrode; and        (9) The electrode and its integral sleeved rod is now introduced axially into the opened end of the lamp tube, while the latter is rotated until the annular bead makes contact with the end of the lamp tube. Both are reheated until the GS becomes molten and can be worked, using a carbon tool, so as to cause the ring of GS defining the open end of the lamp tube to become merged with the GS bead on the tungsten rod, and the GS material to become fused into a uniform annular seal.        
A lamp requires a similar arrangement at the opposite end, and the appropriate steps may be repeated at the opposite end of the lamp tube to enable a second electrode to be sealed in a similar manner into the opposite end.
Final assembly of a lamp involves evacuation of the lamp tube assembly and usually the introduction of a specific gas, usually at low pressure, via the side tube, which is then finally closed off and sealed by heating.